This invention relates to apparatus for assisting or forcing a person to breathe, and, more particularly, to a ventilator system that is flexible in operation and easily controlled by a user.
In many cases the discomfort of critically ill persons can be eased, and their recovery hastened, by a proper program of breathing assistance supplied by a device termed a "ventilator". In simplest terms, the ventilator either forces pressurized gas into the lungs (e.g., a positive-pressure ventilator) or expands the chest cavity of the patient to draw gas into the lungs (e.g., a negative-pressure ventilator) under a selectable schedule of gas composition, pressure, and flow pattern. While negative-pressure ventilators enjoyed a degree of popularity in the past, their use has been largely replaced by positive-pressure ventilators, and the present invention relates to such positive-pressure ventilators.
The ventilator typically includes a compressor that supplies pressurized air, or the ventilator may operate from hospital pressurized air and oxygen lines. The gas is provided to the patient for inhalation according to a prescribed schedule, such as, for example, a specific pressure profile or a specific gas volume delivery profile with time. The inhalation gas flows to the patient and into the lungs. Many ventilators can be adjusted to either force breaths or respond only to the patient's attempts to breathe and assist in that breathing, or operate in some more complex pattern.
The exhaled gas that flows from the patient may also be controlled. For example, in some cases it has been found useful to maintain the exhaled gas under positive pressure, and the ventilator provides a positive end expiratory pressure (PEEP) mechanism for that purpose. A conventional PEEP mechanism restricts exhalation by closing off the path for exhaled gas flow when airway pressure drops below the pre-set PEEP level.
A primary consideration in the design of ventilators is safety, in terms of both avoiding adverse effects of apparatus failure and ensuring that the ventilator aids the patient's own efforts to breath. The ventilator must cooperate with the efforts of the patient to breathe, and indeed the ventilator must permit the patient to be "weaned" from full ventilator dependence to self sufficiency. Ideally, the ventilator should never work against the patient's own efforts. Instead, the ventilator may provide aid for patient-induced breaths, may induce ventilation without the assistance of the patient, or may accomplish a combination schedule of permitting or assisting the patient with self-triggered breaths and then ventilating without patient assistance between the patient's own breaths.
Respiratory therapy has developed into a complex field as more has been learned about the beneficial effects of proper ventilation in a variety of circumstances. Doctors are trained to understand the requirements of proper gas supply to the lungs, and to determine a proper schedule of patient ventilation that is preferred in particular types of cases. For example, the patient with emphysema normally requires quite a different ventilation schedule than the patient recovering from chest surgery.
Early ventilators used all-pneumatic systems for control of gas flow, gas blending, breath rates, patient breath assistance, and pressure control. These systems provided little monitored data and few (or no) alarms and therefore relied heavily on the skill and diligence of the operator to establish and maintain ventilation parameters. Later generations of ventilators contained electronic circuits which provided more precise control of timing parameters such as breathing rates and inspiratory time, and had pressure and flow measuring devices to provide displays of monitored data and to facilitate alarm activations if patient airway pressures, breath-to-breath gas volumes, or frequency of breaths were outside of user-established limits.
The current generation of ventilators use microprocessors to control most of the parameters of ventilation and contain pressure and flow measurement transducers which provide electrical data (via analog-to-digital converters) to the microprocessors for display of monitored parameters and for alarm activations. These microprocessor based ventilators, as compared to previous generations, may have improved flow and pressure control accuracy, may display data in graphic form and present additional data based upon mathematical manipulations of pressure and flow data, and may offer improved safety features. A main advantage of microprocessor based ventilators is the ability to add new features by changing only the memory integrated circuits (usually EPROMs) containing the software programs.
One major disadvantage of some current designs of microprocessor based ventilators is the complexity of the user interface. Typically, ventilator parameters are either input by control knobs, with one knob for each parameter setting as in the prior electro-mechanical ventilators, or by keyboard entry. Parameter settings are typically displayed on seven-segment type displays, either continuously, upon selection, or sequentially in ticker-tape fashion. Likewise, monitored data is displayed on seven-segment type displays. Because of space limitations on the control panel, not all monitored data can be displayed at the same time, and the desired data must be selected for display by selection switches.
As new features and new ventilating modes are added, the complexity of operation increases because the existing controls and display areas must be burdened with the requirement of facilitating input and display of the new features. The microprocessor controlled ventilator also tends to be more costly than previous generations because of the need for designing unique pneumatic control and monitoring devices which are controllable by the microprocessor as well as the need for a non-volatile memory for storage of ventilation parameter and alarm threshold settings. The exhaled gas measurement transducer as an example has traditionally been difficult to design and very expensive to build because of the requirements of accurate flow measurement with very small pressure drops.
Another problem with microprocessor based systems is their susceptibility to AC power line noise. Voltage spikes and other forms of electrical noise can pass through conventional power supplies and interfere with the microprocessor's ability to access the program code from ROM memory and in reading and writing of data to RAM memory. Momentary drop-outs of power line voltage can cause the ventilator power supply's regulated voltages to drop below the operating limits of the integrated circuits, with unpredictable consequences. Power supplies for microprocessor based ventilators must therefore be designed to be immune to all forms of power line noise. Such immunity is normally accomplished with elaborate line voltage spike suppressors and EMI filters.
Conventional ventilators are without battery back-up power supplies and therefore must be designed to shut down in a safe manner with alarms activated if the line voltage momentarily drops below acceptable limits. These requirements are difficult to achieve, and it is nearly impossible to verify that the system is immune to all forms of electrical noise and all durations of power line drop-outs. Hospital power is always backed up by a motor-generator which starts up when the power line drops out. The switch-over in power sources which occurs during a power line blackout or brownout can cause momentary voltage dropouts, voltage spikes, power line frequency shifts, etc., which in turn can cause microprocessor (and other electronic) based systems to fail or shut down. In addition, occasionally the back-up power systems themselves fail, causing all electronic systems in the hospital which are not battery backed to shut off, thereby causing a hazardous situation for the patients.
Thus, there is a continuing need for a ventilator system that is easier for hospital personnel to use, less expensive, and more tolerant of fault situations such as powerline problems. The ventilator must be a readily controlled, convenient apparatus that supplies the required ventilation conditions with minimal chances of error, either in setting the ventilation conditions or in meeting the set schedule. The present invention fulfills this need, and further provides related advantages.